The present invention relates to a sidelight type light source device and an image display device using the sidelight type light source device.
Liquid crystal display devices are widely used as image display devices for personal computers and other types of monitors. In general, a liquid crystal display device has a backlight for providing a planar light source for illumination. The backlight is disposed on a rear surface of a liquid crystal display panel. The backlight irradiates the rear of the liquid crystal surface in a manner that the liquid crystal surface, having a specified expansion, has a uniform brightness over its surface, thus uniformly displaying an image in the liquid crystal. For the backlight described above, a fluorescent lamp is generally used as the stick-shaped light source. The fluorescent lamp may be a hot cathode or cold cathode type. The fluorescent lamp provides a linear stick-shaped light source. The light from the fluorescent lamp must irradiate the entire surface of the liquid crystal display panel. Two types of the backlights have been used in the prior art. The two types include a direct backlight and a side backlight (edge light).
The direct backlight includes a fluorescent lamp under a liquid crystal display panel with a dimmer plate and a diffusion plate on the fluorescent lamp.
The side backlight places a fluorescent lamp along one or both sides of a light guide plate made of a transparent resin. Light incident on the light guide plate is directed in the direction of the liquid crystal display panel surface by a reflecting unit produced by processing a back surface of the light guide plate. The processing produces light diffusion which directs uniform planar light over the liquid crystal display panel. The side backlight is suitable for a display device in portable equipment such as a notebook personal computer because it can be thinner compared with a display device using direct backlight.
Notebook personal computers continue to progress in reduction of thickness and weight in order to enhance portability. As a consequence, the weight and thickness of liquid crystal display devices, including their backlight, must also be reduced.
Technology has been developed for thinning the light guide plate, which is a principal constituent component of the backlight. This technology continues to satisfy the functions of the light guide plate. Currently, light guide plates having a thickness of about 2 to 3 mm are available. Light guide plates conventionally use acrylic resin which has excellent light transmittance. A typical acrylic resin in this application is, for example, polymethylmethacrylate. Polymethylmethacrylate has a refractive index of 1.49 and a critical reflection angle: 42 degree.
The linear-source fluorescent lamp is disposed along an incident surface of the light guide plate in a body called a reflector which guides light from the fluorescent lamp to the light guide plate efficiently.
Referring to FIGS. 9 and 10, a light guide plate 1 includes an incident surface 1a, a light-emitting surface 1b and a reflection surface 1c. A fluorescent lamp 2 is disposed along the incident surface 1a. A reflector 3 surrounds the fluorescent lamp 2. Rubber blocks 4 made of, for example, silicon rubber, are attached to both ends of the fluorescent lamp 2. Due to the presence of the rubber blocks, the light source includes a light-emitting portion denoted by EA in FIG. 10 and non-light-emitting portions which are the portions covered by the rubber blocks 4. The rubber blocks 4 are disposed in contact with the incident surface 1a of the light guide plate 1. The rubber blocks 4 protect electrodes (not shown) at the ends of the fluorescent lamp 2, and prevent heat from the fluorescent lamp 2, which may rise to about 100 centigrade, from being conducted directly to the light guide plate 1. A high voltage, ranging from several hundred volts to more than a thousand volts, is applied to the electrodes during the operation. The rubber blocks 4 insulate this high voltage.
The light irradiated from the fluorescent lamp 2 is incident on the incident surface 1a of the light guide plate 1. The incident light travels through the light guide plate 1 without leakage to the outside because it meets all of the reflection conditions in the light-emitting surface 1b and the reflection surface 1c of the light guide plate 1 according to Snell""s law. However, as seen with reference to FIG. 10, the end portions (corner portions) in the width direction of the sides of the light guide plate 1 in which the incident surface 1a is formed contact the non-emitting portions covered by the rubber blocks 4. Accordingly, the light irradiated from the fluorescent lamp 2 is not incident on the corner portions of the light guide plate 1.
Referring now to FIG. 11, light incident on the light guide plate 1 does not expand beyond an angle xcex8 of a specified degree or more from a perpendicular to the incident surface 1a due to a difference of refractive indexes of air and the light guide plate 1. This angle is determined by a material constituting the light guide plate 1. In the case of the above-described polymethylmethacrylate, the angle xcex8 is 42 degrees. Accordingly, the light incident from the incident surface 1a does not reach the left (or right) corner portion of the light guide plate 1, which is shown by hatching. Actually, the corner portion is not completely dark since some light is diffused and reflected in the light guide plate 1 to enter the corner portion. However, the portion shown by hatching is relatively dark compared with the other portion, thus producing an unevenness in brightness.
Various technologies for solving the above-described unevenness in brightness have been heretofore proposed. For example, technologies addressed to this problem are disclosed in the gazettes of Japanese Patent Laid-Open Nos. Sho 63 (1988)-33702 (Japanese Patent Publication No. Hei 7 (1995)-107567), Hei 6 (1994)-317796 and Hei 9 (1997)-231822.
In the technology disclosed in the gazette of Japanese Patent Laid-Open No. Sho 63 (1988)-33702, light incident on a central portion of the incident surface is reflected to the end portions in the width direction of the light guide plate by providing a critical angle reflection portion on the incident surface of the light guide plate.
The gazette of Japanese Patent Laid-Open No. Hei 6 (1994)-317796 proposes to provide a dot-shaped diffuse reflection layer on a surface (reflection surface) opposite the light-emitting surface of the light guide plate. This technology aims to supply the light incident on the light guide plate to the corner portion after being subjected to diffuse reflection on the diffuse reflection layer.
The gazette of Japanese Patent Laid-Open No. Hei 9 (1997)-231822 proposes a technology in which an illumination light introducing unit is placed between the fluorescent lamp and the light guide plate. The illumination light of the light source is expanded to an end portion side of the light source by the illumination light introducing portion to be guided to the light guide plate.
An additional requirement for liquid crystal display devices for portable equipment, such as notebook type personal computers, is narrowing of the picture frame. The picture frame is the peripheral non-image display region surrounding the image display region in the liquid crystal display panel. When the liquid crystal display device is miniaturized, if the area of the picture frame remains the same, the image area of the liquid crystal display region is reduced. In this event, the value of miniaturization is also reduced. If the area of the picture frame is reduced, the consequent reduction in the non-image display region permits the achievement of a larger image display region within a reduced overall dimension.
The above-described shortage of light at the corner portion of the light guide plate was not a great problem before the demand for a narrower picture frame. However, now that the picture frame must be narrower and the image quality must be improved, the problem of the shortage of light at the corner portions of the guide plate is now a serious problem.
Referring now to FIG. 12, a light guide late 1 has an image display region A on which is stacked a picture frame B. The picture frame B is a typical wide picture frame. The portion of the light guide plate 1 that lacks brightness is shown by hatching. It will be noticed that the entire hatched area is in the non-image area covered by the wide picture frame B. Thus, brightness degradation in the hatched area is not of concern.
Referring to FIG. 13, a light guide plate 1 has an image display region A surrounded by a picture frame B. The picture frame B is a typical narrow picture frame. Because of the narrow picture frame B, some of the area which is lacking in light is exposed to view. The area with degraded light is shown by hatching. It will be noticed that the narrower picture frame B places some of the area of degraded light in the image display area A. Thus, the areas of the light guide plate 1 having degraded light becomes a problem with the narrower picture frame B. The uneven brightness in the image display area A adversely affects the brightness and uniformity of a displayed image.
The above-described technologies disclosed in the gazettes of Japanese Patent Laid-Open Nos. Sho 63 (1988)-33702 (Japanese Patent Publication No. Hei 7 (1995)-107567), Hei 6 (1994)-317796 and Hei 9 (1997)-231822 are respectively effective for solving the unevenness in brightness. However, as shown below, there is still room for further improvement.
In the technology disclosed in the gazette of Japanese Patent Laid-Open No. Sho 63 (1988)-33702, a critical angle reflection portion is provided on the incident surface of the light guide plate. Disclosed as a concrete aspect of the critical angle reflection portion is either a slit formed on the incident surface of the light guide plate or a material filled in the slit, which has a refractive index higher than that of the light guide plate. The light guide plate is typically made by injection molding requiring extremely strict control for forming the slit. It is not simple to form the slit with high accuracy, particularly on a current light guide plate having a thin plate thickness of about 2 to 3 mm. A method is conceivable, where a light guide plate having no slit formed thereon is obtained by injection molding, and thereafter, the slit is formed by a machine process. However, even in this case, it is not yet simple to form a desired slit with high accuracy. In addition, adding a machine process after injection molding adds to the manufacturing cost.
The technology disclosed in the gazette of Japanese Patent Laid-Open No. Hei 6 (1994)-317796 is effective in that a light diffuse reflection layer can be formed relatively simply, and is appreciated in that the quantity of light at the foregoing corner portion of the light guide plate is increased. However, according to examination of the inventors of the present invention, the quantity of light at the corner portion must be further increased in order to meet the recent demand for the higher image quality.
The technology disclosed in the gazette of Japanese Patent Laid-Open No. Hei 9 (1997)-231822 increases the quantity of light at the corner portions by adding an illumination light introducing portion between the fluorescent lamp and the light guide plate. However, the presence of the illumination light introducing portion increases the size of the side backlight unit. Accordingly, this technology cannot meet the demand for miniaturization of the liquid crystal display device, specifically, the demand for the picture frame narrowing.
In consideration of the foregoing problems, the object of the present invention is to provide a sidelight type light source device capable of solving the shortage of light at the corners of the light guide plate at low cost while meeting the demand for narrowing the picture frame.
Another object of the present invention is to provide a light guide plate for use in the sidelight type light source device described above.
Another object of the present invention is to provide an image display device capable of solving the unevenness in brightness due to the shortage of light at the corners of the display.
The present invention defines a gap between each of the rubber blocks 4 and the light guide plate 1. Specifically, a gap between the rubber block 4 and the light guide plate 1 enables the light from the fluorescent lamp 2 to be directly incident on the corner portion of the light guide 1. In order to define the gap between the rubber block 4 and the light guide plate 1, the fluorescent lamp 2 having the rubber blocks 4 thereon is spaced from the light guide plate 1. However, since such spacing results in enlarging of a size of the sidelight type light source device including the fluorescent lamp 2, the spacing is contrary to the desire for picture frame narrowing. Therefore, the inventors of the present invention define a gap between the rubber block 4 and the light guide plate 1 by appropriately modifying the shape of the incident surface 1a of the light guide plate 1. Thus, the light from the fluorescent lamp 2 is incident on the corner portion of the light guide plate 1. Such modification of the light guide plate in shape is performed while satisfying the demand for picture frame narrowing at a low cost.
Accordingly, the sidelight type light source device of the present invention is a sidelight type light source device, comprising: a light guide plate including an incident surface on which light irradiated from a light source is incident and a light-emitting surface for emitting the light incident on the incident surface therefrom, a stick-shaped light source including a light-emitting portion having a length shorter than a width dimension of the incident surface and non-light-emitting portions is located on both end portions of the light-emitting portion, the stick-shaped light source being disposed along the incident surface of the light guide plate. The sidelight type light source device is characterized in that the foregoing incident surface includes a first incident surface located on a central portion in a width direction of the incident surface and second incident surfaces located on both end portions in the width direction, the second incident surfaces being continuous with the first incident surface, and a light introducing route is provided between the non-light-emitting portions of the light source and the second incident surfaces.
According to the sidelight type light source device of the present invention, even in a light source having non-light-emitting portions because of the above-described rubber blocks 4 attached thereto, since the light introducing route is formed between the non-light-emitting portion and the second incident surface, the light from the light-emitting portion can pass through the light introducing route, and is incident on the second incident surface. The second incident surfaces are located in the vicinity of the end portions of the light guide plate, whereby the quantities of light at the concerned portions is increased.
In the sidelight type light source device of the present invention, the second incident surface can take various forms. The second incident surface is a slant surface slanting relative to the first incident surface. Slanting the second incident surface defines a gap between the light guide plate and the non-light-emitting portion without requiring a large spacing between the light source and the light guide plate. This gap constitutes the light introducing route. In order to form the second incident surface consisting of a slant surface, the corner portion of the light guide plate may be removed by any suitable process such as, for example, cutting, grinding, beginning with a rectangular light guide plate having a rectangular cross-section. The present invention does not limit a forming method thereof.
Moreover, in the sidelight type light source device of the present invention, where the second incident surface is slanted with respect to the first incident surface, it is desirable that the second incident surface be constituted of a non-mirror surface. Usually, the incident surface of the light guide plate includes a mirror surface. However, by forming the second incident surface as a non-mirror surface, an incident angle of light on the second incident surface is increased. This ensures that the quantity of light directly incident on the second incident surface is increased.
In a sidelight type light source device of the present invention, the foregoing second incident surfaces can also be formed on parts in the thickness direction of the light guide plate. For example, the second incident surface consisting of the foregoing slant surface may be formed across the entire region in the thickness direction of the light guide plate, that is, from the light-emitting surface to the reflection surface. The second incident surface may be formed only in a range having a specified height from the reflection surface. In this way, the area of the surface on which the light-emitting surface is formed is not reduced. Accordingly, a wider light-emitting surface is produced.
The present invention provides a sidelight type light source device, comprising: a light guide plate including at least one incident surface and a light-emitting surface for emitting light incident on the incident surface therefrom, a stick-shaped light source including a light-emitting portion with non-light-emitting portions at the ends of the stick-shaped light source. The stick-shaped light source is disposed along the incident surface of the light guide plate. The incident surface includes a first incident surface facing the light-emitting portion of the light source and second incident surfaces facing the non-light-emitting portions of the light source. The second incident surfaces are set back from the first incident surface taken as a reference in an incident direction of light from the light source.
According to the sidelight type light source device of the present invention, since the second incident surfaces are set back from the first incident surface taken as a reference in the incident direction of the light from the light source, the light from the light-emitting portion of the light source can reach the second incident surfaces as well. Accordingly, the sidelight type light source device of the present invention contributes to the prevention of uneven brightness, especially in the corners of the light guide plate.
In the present invention, while the incident surface is formed on the entire region in the width direction of the light guide plate, the incident surface is also formed on a specified range excluding the end portions in the width direction. For example, a form corresponds thereto, where concave portions thereof are formed adjacent to the ends. The surfaces of the concave portions are the second incident surfaces. According to the form, since the second incident surfaces are formed, the light incident thereon is securely made incident on the light guide plate without diverting light from the light guide plate.
The present invention provides an image display device, to which the above-described sidelight type light source device of the present invention is applied. Specifically, the image display device of the present invention is an image display device, comprising: an image display panel including an image display region and a non-image display region surrounding the image display region; and a backlight unit for irradiating the image display panel. The foregoing backlight unit is disposed on a back surface of the image display panel. The foregoing backlight unit includes a light guide plate having incident surfaces on which light irradiated from a light source is incident and a light-emitting surface for emitting the light incident on the incident surfaces therefrom. The light source has a light-emitting portion with non-light-emitting portions located on the ends of the light-emitting portion. The foregoing light source is disposed along the incident surfaces of the light guide plate. A light introducing route is defined between the non-light-emitting portions of the light source and the incident surface facing the non-light-emitting portions.
In the image display device of the present invention, the foregoing incident surface includes a first incident surface located on a central portion in a width direction of the incident surface and second incident surfaces located on the end portions in the width direction. The foregoing second incident surfaces are continuous with the first incident surface. The second incident surfaces are set back from the first incident surface taken as a reference in an incident direction of light from the light source. This forms the light introducing route.
Since the light introducing route is a gap defined between the light source and the light guide plate, the light introducing route does not constitute the light-emitting surface. If the light introducing route is located within an area corresponding to the image display region of the image display panel, the quantity of light of that portion of an image is significantly reduced. Accordingly, it is desirable that the light introducing route be located within the area corresponding to the non-image display region of the image display panel.
As a concrete application example of the image display device of the present invention, a liquid crystal display device is cited. However, one skilled in the art will recognize that the present invention can also be applied to other types of image display devices without departing from the spirit and scope of the invention.
The present invention also provides a light guide plate for use in the sidelight type light source device and the image display device of the present invention, which have been described above.
Specifically, the light guide plate of the present invention comprises: an incident surface upon which light irradiated from a light source is incident. A light-emitting surface emits light therefrom that is incident on the incident surface. A reflection surface reflects light incident on the incident surface toward the light-emitting surface. The foregoing reflection surface is opposite the light-emitting surface. The foregoing incident surface includes a first incident surface located on a central portion in a width direction of the incident surface, and a second incident surface set back from the first incident surface taken as a reference in an incident direction of the light from the light source. The second incident surface is a non-mirror surface.
In the light guide plate of the present invention, the foregoing second incident surface is formed in a specified range from the reflection surface in a thickness direction of the light guide plate. Moreover, the second incident surface may also be formed as a slant surface extending from a corner portion of the incident surface side of the light-emitting surface toward the reflection surface.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.